The invention relates to spirally wound lithium cells having an anode comprising lithium and a cathode comprising a manganese dioxide with a separator therebetween.
Primary (non-rechargeable) electrochemical cells having an anode comprising lithium are known and are in widespread commercial use. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in a nonaqueous solvent. The cells are referenced in the art as primary lithium cells (primary Li/MnO2 cells) and are generally not intended to be rechargeable. Alternative primary lithium cells with lithium metal anodes but having different cathodes are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2). These cells are commonly in the form of button cells or cylindrical cells having about ⅔ the height of a conventional AA size alkaline cell. (Alkaline cells as referenced herein shall be understood to be conventional commercial alkaline cells having an anode comprising zinc, a cathode comprising manganese dioxide, and an electrolyte comprising potassium hydroxide.) The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. Primary lithium cells are in widespread use as a power source for many conventional photographic flash cameras, which require operation at higher voltage and at higher power than is supplied by individual alkaline cells.
Primary lithium cells (other than button cells) are conventionally formed of an electrode composite comprising an anode formed of a sheet of lithium, a cathode formed of a coating of cathode active material comprising manganese dioxide on a conductive metal substrate (cathode substrate) and a sheet of electrolyte permeable separator material therebetween. The separator sheet is typically placed on opposite sides of the lithium anode sheet and the cathode sheet is placed against one of the separator sheets, thereby separating the anode and the cathode sheets. The electrode composite is spirally wound and inserted into the cell casing, for examples, as shown in U.S. Pat. No. 4,707,421. The cathode substrate is typically a stainless steel expanded metal foil. A portion of the anode sheet is typically electrically connected to the cell casing which forms the cell""s negative terminal. The cell is closed with an end cap which is insulated from the casing. The cathode sheet can be electrically connected to the end cap which forms the cell""s positive terminal. The casing is typically crimped over the peripheral edge of the end cap to seal the casing""s open end. The primary lithium cell is typically provided with PTC (positive thermal coefficient) device located under the end cap and connected in series between the cathode and end cap. Such device protects the cell from discharge at a current drain higher than a predetermined level. Thus, if the cell is drained at an abnormally high current, e.g., higher than about 2 Amp, the PTC device expands and heats causing its resistance to increase dramatically, thus shutting down the abnormally high drain.
The primary lithium cell is a nonaqueous cell. The manganese dioxide powder used to form the cathode active material can be conventionally heat treated at temperatures of between about 200-350 xc2x0 C. in vacuum as taught in U.S. Pat. No. 4,133,856 (Ikeda). It is preferable to heat the MnO2, for example, electrolytic MnO2 (EMD) in two steps once to temperatures above 250 xc2x0 C. to drive off non-crystalline water during which step gamma MnO2 is gradually converted to gamma-beta structure. The heated MnO2 can then be heated at higher temperatures between 250 and 350 xc2x0 C. as described in U.S. Pat. No. 4,133,856 prior to insertion of the MnO2 into the cell. The treatment results in better cell performance and higher capacity. The second heating helps to prevent electrolyte leakage. The treated MnO2 is mixed with suitable binders, for example, tetrafluoroethylene (Teflon) binders, and conductive agents, for example, carbon black and graphite. The cathode mixture can be coated onto a metallic substrate such as a stainless steel expanded metal foil.
The anode can be formed by coating a layer of lithium on a metallic substrate such as copper. However, it is preferable that the anode is formed of a sheet of lithium without any substrate.
The electrolyte used in a primary lithium cell is formed of a lithium salt dissolved in an organic solvent. Typically, the salt is lithium perchlorate (LiClO4) or lithium trifluoromethanesulfonate (LiCF3SO3). Other salts which are conventionally used include LiPF6, LiAsF6 and LiCF3CO2 and LiBF4. Organic solvents can typically include ethylene carbonate/propylene carbonate (EC/PC) dimethoxyethane (DME), dioxolane, gamma-butyrolactone, and diglyme.
Recently electronic devices, such as digital cameras, fully featured photographic flash cameras, as well as some high power toys and electronic games have appeared in the commercial market. These devices, require sustained load voltages of at the level of between about 2.5 and 3 Volt and demand high power which can be satisfied with the use of primary lithium cells. Although present commercial primary lithium cells can be used to power many of these devices, it is desirable to increase the cell""s capacity where possible in order to make the lithium cell even more attractive for such application.
It is desirable to increase the lithium cell""s capacity (mAmp-hr) for a given cell size, where technically and economically feasible provided that there is no significant sacrifice in the cell""s power output.
The invention is directed to improvements in spirally wound primary lithium cells to increase the electrochemical capacity (mAmp-hr) of a cell of any given size and shape without changing the basic cell chemistry or thickness of the individual sheets comprising the spirally wound electrode assembly therein. The spirally wound electrode assembly within the cell casing is formed by spirally winding an electrode composite comprising an anode sheet of lithium, a cathode sheet comprising a manganese dioxide and a separator of electrolyte permeable material therebetween. The manganese dioxide can include any form of manganese dioxide useful as cathode active material in primary lithium cells, for example, manganese dioxide, heat treated electrolytic manganese dioxide (EMD) and lithiated manganese dioxide. The cell casing, typically a cylindrical metal casing, has an open end and a closed end. After the electrode composite is spirally wound it is inserted into the open end of the metal casing until it comes to rest against the casing closed end. Electrolyte can then be added.
A principal aspect of the invention is directed to forming improved electrical insulation around the spiral electrode assembly of a primary lithium cell. The improved insulation is accomplished by reshaping an edge of the separator layer, as by heat forming, and using said reshaped portion of the separator layer to also function as a continuous electrical insulation layer between the positive cathode sheet and the cell casing, particularly at the closed end of the casing.
An aspect of the invention involves first forming a spirally wound electrode assembly with the bottom edge of the wound separator sheet extending beyond the bottom edge of the wound anode and cathode sheets thereby exposing the bottom edge of the wound separator sheet. At this stage of formation, the bottom edge of the separator sheet is aligned parallel to the bottom edge of the cathode sheet and therefore there are gaps between each revolution of the bottom edge of the separator sheet and the bottom edge of the cathode sheet. (The term xe2x80x9cbottom edgexe2x80x9d, as used herein, is defined as that edge of the spiral electrode assembly which abuts the casing closed end when the spiral electrode assembly is inserted into the casing.)
In accordance with a principal aspect of the invention a heat source, preferably a heated platen, can be applied to the exposed bottom edge of the separator sheet extending from the bottom of the spirally wound electrode assembly. The separator sheet is electrolyte permeable and heat deformable, preferably of thermoplastic material. A desirable electrolyte permeable sheet having such properties is microporous polypropylene. Alternatively, the separator can be of microporous polyethylene or laminates of polyethylene and polypropylene. The exposed bottom edge of separator softens or melts upon contact with the heat source thereby reshaping the edge by thermoforming. When the heat source is removed, the bottom edge of the separator sheet, upon cooling, solidifies into a continuous (thermoformed) separator membrane which covers the exposed bottom edge of the cathode sheet. Thus, when the electrode assembly is inserted into the cell casing the continuous (thermoformed) separator membrane forms a continuous electrical insulation layer between the bottom edge of the cathode sheet and the inside surface of the closed end of the casing.
Since the continuous separator membrane also lies flush against the bottom edge of the cathode sheet, void space normally present between the bottom of the spirally wound electrode assembly and the closed end of the casing is eliminated. (The term xe2x80x9cvoid spacexe2x80x9d as used herein is intended to mean space not occupied with electrochemically active anode or cathode material.) Also the formation of such a continuous separator membrane covering and electrically insulating the bottom edge of the cathode sheet makes it possible to eliminate the electrical insulating disk which is normally inserted between the bottom edge of the electrode assembly and the inside surface of the closed end of the casing. The space saved by eliminating such void volume can now be utilized by increasing the amount of anode and cathode active material in the cell, for example, by making the electrode sheets wider thereby increasing cell capacity.